Generational shape shifting: changes in egg shape and size between sexual and asexual generations of a cyclically parthenogenic gall former

Successive generations of multivoltine species experience selection specific to the spatiotemporal environments encountered that may lead to adaptive divergence in reproductive traits among generations. To compare reproductive effort within and between generations, appropriate volumetric models, selected on the basis of the analysis of egg shape, are required to estimate the sizes (volumes) of individual eggs. We assessed the shape and estimated the volume of individual eggs produced by the temporally and spatially segregated sexual and asexual generations of the gall former, Belonocnema treatae Mayr (Hymenoptera: Cynipini: Cynipidae). Egg shape, indexed as the difference between the polar and equatorial axes of the ellipsoidal eggs, was independent of egg size, but differed between generations. The relationship of egg shape and female body size within and between generations confirmed that egg shape is an intrinsic property of each generation. Generational differences in egg shape then informed the selection of volumetric models to estimate egg size. We modeled asexual generation eggs as both spheres and prolate spheroids, and sexual generation eggs as both cylinders and prolate spheroids. Choice of volumetric model changed estimates of egg size within the asexual generation by 23% and within the sexual generation by 50%. Comparisons between generations based on the above models produced estimated differences in egg volume that ranged from 16 to 114%. In both generations, a prolate spheroid was the most parsimonious model of egg volume. Based on this model, sexual generation eggs averaged 43% larger than asexual generation eggs. The increased size of sexual eggs was achieved via conservation of the egg’s equatorial axis and elongation of the polar axis. The shift in egg shape between sexual and asexual B. treatae is the first documented dimorphism in an egg characteristic expressed between generations of a cyclically parthenogenic organism.